The present invention relates to a device for determining a malfunctioning of an electric-motor-assisted power steering system of a motor vehicle.
An electric-motor-assisted power steering system is known and described in Japanese Patent Laid-Open Publication No. 61-98670. The known power steering system of this type has an assisting motor and a control circuit. The control circuit controls the rotational direction and the electric current of the motor to rotate the motor for assisting the steering operation of the steering system, on the basis of a sum of a first motor current command value in an assisting direction and of a second motor current command value in the opposite direction. The first current command value is produced in accordance with a steering torque generated by the steering wheel, and the second current command value is produced in accordance with the steering angle.
In the steering system of the above described type, the steering force required for increasing and decreasing the steering angle is reduced by the assisting motor. However, when there occurs a malfunctioning of a steering torque sensor or a steering angle sensor, the steering assisting force becomes excessively large so that it becomes impossible for the driver to maneuver the steering wheel as he wants.
The steering torque sensor has a substantially fixed operating range so that it is relatively easy to determine whether the steering torque is within the operating range. On the other hand, in the case of a steering angle sensor using a potentiometer, it is relatively easy to determine whether a malfunctioning thereof is due to damage of an electric wiring connected to the potentiometer, whereas it is considerably difficult to determine a malfunctioning of the steering angle sensor unless the sensor generates an erroneous output signal of an apparently abnormal magnitude.